A popular thin-film photovoltaic technology is called CIGS, which refers to a p-type semiconductor photon-absorber layer containing at least Copper, Indium, Gallium, and Selenium having the ability to absorb solar radiation and generate electron-hole pairs. In a typical CIGS photovoltaic cell, a Copper-Indium-Gallium-diSelenide layer operates with a heterojunction partner layer to generate a photocurrent when exposed to light. Additional layers, such as a substrate, top and bottom contact layers, passivation layers, and metallization, may be present in the cell for structural rigidity, to collect the photocurrent, and protect the cell.
CIGS semiconductor thin film can be created by a variety of processes, both in vacuo and ex vacuo in nature. Deposition methods such as sputtering, co-evaporation, and combinations of sputtering and evaporation performed in vacuo have produced CIGS photon absorber layers with high demonstrated performance, but traditional means for fabricating the CIGS film are perceived as slow and prone to defects. Both sputtering and evaporation may involve a reactive process to create the CIGS alloy film having desired stoichiometry. Slow fabrication speed can lead to high fabrication cost. Defects in a CIGS film can allow recombination of electron-hole pairs thereby reducing cell efficiency. Further, defects may short-circuit part or all of the photocurrent, impairing function of individual cells and monolithically integrated modules having CIGS cells. Other defects can provide a path, or via, to underlying conductive materials, thereby allowing subsequent depositions of electrically conductive material to generate an electrical short. Defects that cause short-circuiting are known herein as short-circuit defects. Defects therefore reduce manufacturing yield and increase fabrication cost.
Some methods of creating a CIGS absorber layer deposit CIGS directly. Other methods deposit precursor sublayers, such as layers of copper, layers of indium and gallium, and layers of selenium, that are reacted in-situ to form CIGS. Delivery of either CIGS or the sublayers can be performed by a single source, or by a plurality of sources. Existing processes require that the cell remain in a deposition zone for a lengthy time to deposit and form a CIGS layer of the desired thickness.
Many defects in CIGS layers initiate at the surface of the underlying contact layer when the elements are initially disposed on the surface; these defects originate at the bottom of the CIGS layer. Defects originating at the bottom of the layer may propagate through the entire layer. Growing CIGS films to the desired thickness without termination can allow these defects to propagate through the thickness of the film; defects extending through the thickness of the film are particularly prone to cause short-circuit defects because later deposited layers may contact layers underlying the CIGS layer.
Traditional in vacuo processing of semiconductor materials is batch-oriented. Substrates and source materials are placed in a chamber, air in the chamber is pumped out, deposition is performed, air is allowed back into the chamber after deposition is completed, and the substrates are moved to further processing stations. In order to reduce cost of photovoltaic cells by increasing the area of cell produced with each pumping cycle of the chamber, there is much interest in roll-to-roll processing. In roll-to-roll processing, substrate of a feed roll is unrolled within the chamber, passed through at least one deposition and reaction zone, and wound onto a take-up roll after passing through the deposition and reaction zone. In roll-to-roll processing, there is economic advantage in maintaining high substrate transport speed through the deposition zone. High substrate speed through a deposition zone while reaching a desired film thickness requires either an extended deposition zone length or a rapid deposition rate of the film.
Increasing deposition rates of the traditional in vacuo CIGS deposition process typically requires larger size or larger quantity of sources, or both, but the basic sequencing of deposition is typically unchanged and propagation of defects through the entire thickness of the CIGS layer may be enhanced at high deposition rates. Defects propagating through the entire thickness of CIGS that cause the short-circuit defects are particularly critical to large-area CIGS modules formed by monolithic integration. Unlike modules made with discrete cells that are sorted to match performance prior to module integration, a monolithically integrated module is processed from a contiguous section of photovoltaic material, and any defect contained therein can severely affect the performance of that module.